VOLUME33, NUMBER10
Photoisomerizations.
Q Copyrighl 1968 b y lhc Amcrrcan Chemical Socicfy
OCTOBER11, 1968
X. The Photochemical Transformations of Alloocimene’ KEVINJ. CROWLEY~
Department of Chemistry, Institulo Venezolano de Investigaciones CientZJicas (I VIC), Apartado 1827,Caracas, Venezuela Received March 7 , 1968 The two previously unknown 4-cis-alloocimenes are described, together with two other new photoproducts of this terpene. Tentative configurational assignments are made to the two new alloocimenes, and those previously made to the 4-trans isomers are reviewed. The other photoproducts are the enallene (3) formed by 1,5hydrogen migration, and the bicyclo[3.1.0]hexene (Sa). The latter was degraded in four steps to 2,2-dimethylsuccinic acid; its structure has been confirmed by other chemical correlations and by spectral considerations. Acid-catalyzed isomerization gives the cyclopentene 18, and pyroysis gives the cyclopentene 16. Hydrogenation of these, to give two known 1,2dimethyl-3-isopropylcyclopentanes,17a and 17b, established the stereochemistry illustrated.
The photochemistry of acyclic trienes has been intensively examined in recent years, and several modes of photoisomerization have been observed. The reversible ring closure to a 1,3-cyclohexadiene is perhaps the most common of these, and alloocimene (1) afforded one of the early examples of this behavior. I n 1962 Fonken3 reported that this terpene is in photochemical equilibrium with the isomeric a-pyronene (2). I n the course of some work in this laboratory on the photoisomerizations of conjugated dienes it was observed that appropriately substituted acyclic dienes could undergo a photochemical 1,5-hydrogen shift.4 The vapor phase photoisomerization of 1,3,5-hexatriene to 1,2,4-hexatriene6 suggested that these 1,s shifts might be extended to conjugated acyclic trienes in general, particularly since this type of reaction can be made photochemically irreversible by the choice of appropriate light sources and filters. Alloocimene appeared to be a suitable material with which to test this hypothesis. Two geometrical isomers of this terpene have been known since 1944,6 and Raman16 u l t r a ~ i o l e t , ~in-~ frared,’~~and mass spectral0 have been recorded. (1) (a) For paper I X , see K. J. Crowley, K. Erickson, A. Eckell, and J. Meinmald, manuscript in preparation. For preliminary communications, see (b) K. -1. Cronley, Proc. Chem. Soc., 17 (1964); (c) K. J. Cromley. Tetrahedron Lett., 2863 (1965). (2) University Chemical Laboratory, Trinity College, Dublin 2, Ireland. (3) G. J. Fonken, ibid., 549 (1962). (4) K . J. Crowley, Tetrahedron, 111, 1001 (1965). (5) R. Srinivasan, J. Chem. Phys., 38, 1039 (1963), and earlier papers. (6) J. J. Hopfield, S. A. Hall, and L. A. Goldblatt, J . Amer. Chem. Soc., 6 6 , 115 (1944). (7) R. T. O’Connor a n d I,. A. Goldblatt, Anal. Chem., 116, 1726 (1954). (8) K. Alder, A. Dreike, H. Erpenbach, and U. Wicker, Ann., 609, 1 (1957); cf. footnote h , Table I. (9) C. Cappas. Ph.D. Thesis, University of Florida, 1962; Dissertation Abslr., 118, 846 (1962). (10) A. F. Thomas and B. Willhalm, Helu. Chim. Acta, 47, 475 (1964).
3679
Their stereochemistries are still questioned” although it is generally agreed that both isomers are trans at the 4,5 double bond (la, lb). Uncertainty about the configuration at the 6,7 double bond has been compounded by the use (sometimes overlooked) of two conflicting terminologies. I n this paper the term alloocimene is applied to all four isomers, and, in accord with the IUPAC ruling,12 l a is referred to as the 4-trans-6-trans isomer. Some authors8J3J4use the IUPAC terminol-
Id
3 (11) “Molecular Rearrangements,” Vol. 11, P. d e Mayo, Ed., Interscienoe Publishers, Inc., New York, N. Y., 1964, p 790. (12) “Handbook for Chemical Society Authors,” The Chemical Society, London, 1960, p 190. (13) F. H. A. Rummens, Rec. Trau. Chim. Paus-Bas, 84, 1003 (1965). (14) M. H . Klouwen and R. ter Heide, J . Chromatog., 7 , 297 (1962).
3680 CROWLEY
The Journal of Organic Chemistry TABLE I LITERATURE ON 4-tTanS-ALLOOCIMENES
Trivial terminology
BP, 'C (mm)
-Ultraviolet
maxima,= mM--
Maleic anhydride adduct (mp, O C )
Structure given
Ref
Alloocimene A Heloocimene B Neoalloocimene Alloocimine (A4Y (-45)" Alloocimene -4 Alloocimene B Alloocimene I Alloocimene I1
89 (20) trans,trans 267 277 288 91 (20) trans,cis 263 272 282 (84) L89.1 (2O)lb 270 279 290 38 lb [88.1 (2O)lb 267 275 286 84 la Low boilingd lb High boilingd la 89 (20) 267 277 288 lb 91 (20) 263 272 282 la Low boilingd la High boilingd lb Low boilingd 3642 lb High boilingd 80-82 la Low boilingd 268 277 287 37-40 lb High boilinga 264 273 282 82 la a All in isooctane, except ref 8, where the solvent is not specified. Alder, et aZ.,8 would appear to have inadvertently exchanged boiling points, densities, and formation temperatures of the two isomers; other workers agree that the higher boilingd isomer has the higher density,flJ,@ lower wavelength ultraviolet maxima,?,$higher-melting maleic anhydride adduct,'6 and higher temperature of formation.9 c A4 and A5 are the trivial designations used by Klouwen.l4 d I n agreement with other workers@s14 it has been found that the lower boiling isomer has the lower retention time on all gas chromatographic columns examined.
I
22
18
14
10 (mini
Figure 1.-Photoproducts of alloocimene (see Experimental Section); the glpc curve was obtained using a squalane capillary column a t 101'. The higher baseline between I C and Id is due t o the progressive decomposition of the enallene (3) in the column to yield these two products.
ogy, the opposite, based on the relationship of like groups, and several investigator^^,^,^^,'^ do not clearly indicate which convention is followed. Configurations have been assigned by various authors on the fallible basis of comparative physical and spectral properties (see Table I). After initially assigning l b to the low boiling isomer,I7 Rummens13 now considers this to be la, on the basis of infrared frequency shifts. Alder, et aZ.,s conclude, from extensive degradations of the maleic anhydride adducts, that the isomer giving the adduct of mp 84" is l a and that giving the adduct of mp 38" is lb. This is supported by the recent work of Jlilks and Lancaster,15 who give nmr spectra of the two adducts and the dissociation constants of the corresponding diacids, both of which indicate that the higher melting adduct (from the higher boiling isomer) is of la, and the lower melting, of lb. The results given below are in accord with these conclusions. When a mixture of the two 4-trans-alloocimenes (10% la, 90% lb) was irradiated, six main photoproducts (IC, Id, 2-5) were observed (see Figure 1). Two of these (ICand Id) disappeared, together with l a and lb, on prolonged irradiation. Products ICand Id were obtained both from such an irradiation product mixture and by pyrolysis of the enallene (3) which is described below. Their nmr spectra are similar, and differ very little from those of (15) J. E. Milks and J. F Lancaster, J . O w . Chem., 80, 888 (1965). (16) Y.-R. Naves, Helv. Chzm. Acta, 49, 1029 (1966). (17) F. H. A. Rummens, Diss. Techn. Hogeschool t e Eindhoven 1963, p 123, as quoted in ref 16.
l a and lb, and their infrared spectra suggest that the only double bonds present are trisubstituted and cis disubstituted. Their ultraviolet spectra suggest that they are both conjugated trienes, although that of Id, in particular, is somewhat unusual,lB perhaps because of a reduced hyperconjugative contribution from the nongeminal terminal methyl group, owing to lack of planarity of the triene system. Hydrogenation of a mixture of IC (81%) and Id (19%) gave only 2,6-dimethyloctane, and pyrolysis gave a-pyronene (see Table 11). These results indicate that both products are 4-cisalloocimenes. This is confirmed by an analysis of the results given in Tables I1 and 111, which also permits the tentative assignments of structure IC to the less volatile of the two photoisomers and Id to the more volatile. These assignments are based on the consensus of earlier workers that, of the two 4-trans isomers of alloocimene, the higher boiling is la. The isomerizations l a + I C and l b + Id are one-step reactions, but l a + Id and l b IC each require isomerization around two double bonds, and should thus be much slower. There is no reason to expect a large difference in the quantum yields of the two single-step reactions, Id should be the two main and thus l a -t IC and l b reactions in the early stages of the irradiation of a mixture of the two 4-trans isomers. It is then apparent from Table 111 that 1b gives rise to the more volatile photoproduct, which is thus Id, and la, the highest boiling alloocimene, gives rise to the less volatile isomer, which is thus IC. The relative rates of formation of these four isomers during the early stages of the irradiation of a-pyronene, which are shown in Table 111, confirm these relationships. The formation of a 4-cis-alloocimene on heating the enallene 3 should take place by a thermal (suprafacia120) -f
-
(18) T h e gas chromatographic behavior of i d is also unusual'* in t h a t it is eluted more rapidly than would be expected for a conjugated triene, and in the large difference in retention index between i d and IC, compared with t h a t between lb a n d 1s (see Figure 1). (19) Thanks are extended to Dr. M.Porter of the Natural Rubber Producers Research Association for this observation. (20) R. B. Woodward and R. Hoffmann, J. Amer. Chem. Soc., 87, 2511 (1965).
ALLOOCIMENE 3681
Vol. 33, N o . 10, October 1968 TABLE I1 THERMAL ISOMERIZATION OF THE ENALLENE (3)" Heating time, min
Temp, ' C
a
Unknown
Unknown
Product composition, % ' (by --pbgc-)l id UnknownC
S
I C
la, b
112-114
1 6 0 72 22 dl e 2 17 1 45 37 d, e 4 16 2 29 53 4e 34 3 8 7 56 4e 0 40 3 0 19 0 38 d, e 260-270 0.25 1 1 18 2 0 17 d, e 57 5 10 2 0 0 78 8 2 d, e 3 In order of elution a The starting material was >93% 3 and contained no contaminants other than IC and Id; see footnote 23. Its high retention time militates against this product being the isopropylidenecyclobutene which from the capillary column (at 68"). might be anticipated in the light of the results of Gil-Av. See E. Gil-Av and J. Herling, Tetrahedron Lett., I (1967). The product was la and lb are slowly formed when the allene is kept at room temperature (50 and 30%, respecnot isolated. d None detected. Id may be explained by participation of the 2,3 double bond; it is also tively, after 2 years). The more facile isomerization IC observed on glpc and is rapidly reversible at 130': cf. E. M. Marvel, G. Caple, and B. Schatz, ibid., 385 (1965).
-
TABLE 111 ALLOOCIMENE AND O F CY-PYRONENE. GASC€IROM.\TOGRAPHIC A N A L Y S ~OSF~THE MAJORINITIAL PRODUCTS
IRR.4DI.iTION" O F
Irradiation time, min
0 2 3 4 6 16 40
6a
0 0.13 0.24 0.4 1.1
2.0 10.4
a
a
Composition, Yold 10
0 4.0 5.0 6.3 8.9 11.3 16.3
0 1.2 1.7 2.0 2.4 4.2 7.8
lb
73 67.3 64.1 62.7 59.4 53.3 40.6
la
-
27 29.8 28.9 28.7 28.1 29.3 25.2
0 0 0 0 0 0 87 0.7 0.5 2.2 76.5 4.5 1 0.4 4.5 1.3 6.4 1.0 71.4 2 0.6 2.0 6.8 7.6 1.4 3 1.0 65.6 8.7 2.7 1.7 62.0 8.0 4 1.4 10.9 4.2 2.8 10.2 6 2.5 51.0 12 4.6 25.8 12.9 5.4 25.4 7.2 As 1% (alloocimene) and 0.4y0 (a-pyronene) solutions in purified hexane (100 ml), under a slow stream of nitrogen. A Vycor filter was used. b A 150 ft x 0.01 in. capillary column, with squalane as stationary phase, was employed a t 140 (alloocimene) and 120' (a-pyronene). Portions (1 ml) of the solutions were removed a t the specified times, and most of the solvent was removed by evaporation before analysis. The products are listed in order of elution. The amount of a-pyronene remained below 1% during the irradiation. When a Corex filter was used the proportion rose, but in no case did its proportion reach the value of 20% given by Fonken.3 The yield of enallene (3) was also low under the conditions used. An impurity (13%) remained unchanged throughout the irradiation.
1,5-hydrogen shift, a very general reaction.z1 Models indicate that steric factors favor the 4-cis-6-trans isomer (IC) rather than Id as the immediate product of this isomerization. Since the less volatile new isomer is the main initial product (see Table 11) this would appear to be IC; the other should be Id. This is in agreement with the above correlations. In contrast, a similar naive approach using the conrotatory photochemical ring opening of a-pyronene to a 4-cis-alloocimene leads to the opposite conclusion. Of the two conrotatory paths available, that leading to IC appears to be sterically favored over that leading to Id. This is because the secondary methyl group of 2 is compressed between the two geminal dimethyl groups in the transition state leading to Id, but not in (21) J. Wolinsky, B. Chollar, and M. D. Baird, J . Amer. Chem. Soc., 84, 2775 (1962).
that to IC. The high l d / l c ratio in the early stages of the irradiation of a-pyronene (see Table 111) indicates that Id, rather than the expected IC, is the initial product. Such steric considerations are probably less reliable when applied to photochemical as opposed to thermal reactions.t2 The thermal (disrotatory) cyclization of ICt,o a-pyronene should occur more readily than Id -* 2, but according to the result,s given in Table I1 the thermal interconversion of IC and I d takes place much more rapidly than cyclization; this reaction cannot be used to further clarify the identkies of IC and Id. The presence of the enallene (3) is readily observed by means of its characteristic infrared maximum at 5.12 p . Its yield was improved when a Corex filter was used, and when the solution was kept cold (0') during the irradiation, but beyond this no att.empt was made to optimize the isomerization conditions. By careful fractional distillattion at room temperature the enallene was isolated with little decomposition. Gas chromatography indicated the absence of contaminants other than IC and ld,z3and its ultraviolet absorption at 270 mp showed it to contain less than 2% alloocimene; it gives one maximum, at 225 mp (e 24,600).z4 It gives a strong infrared band at 865 cm-1 attributedz5 to a cis-disubstituted double bond conjugated with an allenic grouping. Hydrogenation, with the uptake of 3 mol of hydrogen to yield only 2,6-dimethyloct,ane, showed that the enallene possesses the original carbon skelet'on of alloocimene. Its nmr spectrum includes a nine-prot'on complex centered at 6 1.75, indicative of three vinyl methyl groups, and is in accord with two structures, of 2,6-dimethylocta2,3,5-triene (3) and 2,6-dimethylocta-2,4,5-t'riene. The thermal behavior of the allene confirmed the prognosticated structure (3), the two 4-cis-alloocimenes being virtually the only products of mild pyrolysis (Table 11). The formation of these can be explained readily as a 1,j-hydrogen shift,*I whereas the transformation of the 2,4,5-triene to IC or I d requires a (22) R. B. Woodward and R. Hoffmann, ibid., 81, 395 (1965). (23) Owing to its thermal instability t h e purity of the enallene (3) cannot be determined by gas chromatography. It slowly decomposes even a t 68', yielding IC a n d I d ; under optimum conditions, when only about 5% is d e
composed, lc/ld = 10, in agreement with the results in Table 11. (24) This is comparable with the maximum a t 222 m r ( e 18900) for a n analogous triene of the type RCH=C=CH=CHR*S (cis) since the enallene (3) shows only end absorption in the 250-300-mp region; the maximum a t 268 m,u ( e 3,800) reported by these workers for a n analogous compound may be due to the presence of conjugated triene impurity. ( 2 5 ) K. L. Mikolajczak, M. 0. Bagby, R. B . Bates, and I. A. Wolff, J . Ow. Chem., SO, 2983 (1965).
The Journal of Organic Chemistry
3682 CROWLEY
Ozonolysis, followed by oxidation of the ozonide with aqueous methanolic hydrogen peroxide, resulted in a 38% yield of the crystalline keto acid, 7. The carbonyl group gives rise to maxima at 275 mp ( e 45) and at 5.82 p andt he carboxyl group at 5.92 1.1. The nmr spectra of this acid and its methyl ester 8 are in
7, R - H 8, R CH, Figure 2.-Nuclear
magnetic resonance spectrum of 5a (without solvent).
thermal 1,3-hydrogen shift, for which there are no analogies, and which would not be expected to occur under such mild conditions.20
9, R- CH 10, R e CHS
cH302ciP A
12
14
6
An analogous photoisomerization, of ethyl 1,3,5-hexatriene-2-carboxylate to t,he corresponding enallene,26 has been reported; the product undergoes base-catalyzed isomerization to the conjugated enyne. Attempts to bring about a similar isomerization of hydrocarbon 3 using methanolic potassium hydroxide and a saturated solution of trimethylamine in pyridine, yielded only starting material. The second main photoisomer obtained on prolonged irradiation is formed in about 14% yield, and was isolated in 98% purity by careful fractional distillation. It has the bicyclo [3.1.0]hexene structure 5 , which can be surmised solely on the basis of its nmr spectrum. This (Figure 2) shows four methyl groups [one vinyl (6 1.67), one secondary (6 1.05, J = 7 cps), and two tertiary (6 1.04 and 0.82)27]and four one-proton resonances (located partly by spin decoupling) , a t 6 5.26 (i), 2.06 (iv), 1.6 (ii), and 0.96 (iii). Double irradiation experiments showed the vinyl proton (i) to be coupled with the vinyl methyl ( J < 1 cps) and with the proton (ii) at 6 1.6 ( J < 1 cps) and also showed the latter to be coupled ( J = 6 cps) with the 6 0.96 proton (iii). This, in turn, is coupled ( J = 1.1 cps) with the 6 2.06 proton (iv), while saturation of the latter also collapses the secondary methyl doublet and, to some extent, the vinyl methyl resonance. From this the partial structure 6 can be deduced, and 5 follows. The photoproduct shows an ultraviolet maximum at 212 mp, and infrared maxima at 3.29, 6.09, and 12.33 p which are consistent with the presence of a trisubstituted double bond conjugated with a cyclopropane ring. (26) H. Prinrbach a n d E. Druckrey, Tetrahedron Lett., 2959 (1965).
(27) The chemical shifts for these two methyl groups are close to those (6 0.8, 1.07) reported for the similar methyl groups in Aa-carene by S. P. Acharya, ibid., 4117 (1966).
11
13
15
accord with the assigned structures. Sodium hypobromite oxidation of the keto acid to the crystalline dicarboxylic acid 9 further confirmed the presence of the methyl ketone group. The acid 9 yielded an anhydride (1l), also crystalline, which showed infrared maxima at 5.57 and 5.71 p, indicative of a glutaric, rather than a succinic, anhydride. Degradation of the photoproduct to a known compound was achieved by base-catalyzed fission of the cyclopropane ring of the keto ester 8. This is an extention of the work of Widmark2* and of Crombie and coworkersz9 on the base-catalyzed ring opening of cis- and trans-methyl homocaronates to 3,3-dimethylbutene-1,4-dicarboxylic acid. When the ester 8 was treated with methanolic potassium hydroxide and the resulting acid mixture was reesterified, gas chromatography indicated starting material (25%) and a new ester (73%) and yielded the latter in the pure state. I n agreement with the anticipated structure (12),the product has ultraviolet absorption [A, 292 mp (e 83), 228 mp (e 13,900)] corresponding to a mono- or disubstituted a,P-unsaturated ketone, and infrared maxima indicative of the same group (5.99 p ) , of a saturated ester (5.76 p ) , and of a carbon-carbon double bond (6.10 p ) . The structure (12) is completely confirmed by the nmr spectrum. Ozonolysis of this unsaturated keto ester (12) followed by performic acid oxidation yielded the known 2,2-dimethylsuccinic acid. Hydrolysis of the same keto ester yielded the free acid; since this could not be crystallized, it was oxidized directly with sodium hypobromite and gave the crystalline dicarboxylic acid 13 which showed Amax 212 mp ( E 12,000) as expected3Qfor a disubstituted alp-unsaturated acid and gave infrared and nmr spectra in agreement with the structure 13. (28) G. Widmark, Ark. Kemi, 11, 195 (1957).
(29) L. Crombie, J. Crossley, and D. A. Mitchard, J. Chem. Soc., 4957 (1963). (30) A. T. Nielsen, J. Ore. Chem., PB, 1539 (1957).
ALLOOCIMENE3683
Vol. $3, No. 10, October 1968 I n the alkaline ring fission of the keto ester 8, the keto group stabilizes the intermediate anion (14), while the ester groupling stabilizes the resultant acyclic anion (15). That the free carboxylate ion is less efficient in stabilizing the corresponding anion is shown by the slower rate of fission when the keto acid 7 is subjected to the same alkaline conditions: the main product is the same, but is formed about twenty times more slowly. On the other hand similar treatment of the diester (10) resulted in no detectable amount of the acyclic diacid (13), while cis-homocaronic acid, the homologous diester without the a-methyl group, undergoes ring opening to the extent of %younder the same c~nditions.~gThis illustrates the marked destabilizing effect of an a-methyl group on the intermediate anion. With the structure of the photoproduct established as 5, its stereochemistry can nom be considered. The reversible formation of the anhydride 11 shows that the cyclopropane-cyclopentene ring fusion is cis; furthermore no example of a tmns-fused bicyclo [3.1.0]hexane is known. Of the two possible cis-fused structures, Sa and Sb, the former was favored,lCsince JlSs= 1.1 cps, which suggests31 that the two protons are approximately at right angles. However, since the relevant bond angles are not close to tetrahedral, such a conclusion should be considered as very tentative ; it was, nevertheless, confirmed by the following chemical correlations. To determine the configuration of the 4-methyl group relative to the cyclopropane ring, reactions were sought in which the cyclopropane ring is opened without the risk of inversion of the resulting isopropenyl group, while the cyclopentene ring remains intact. This was achieved in two ways, by pyrolysis and by acid catalysis. Thermal l15-homodienyl hydrogen shifts are now widely r e ~ o g n i z e d and , ~ ~ would be expected to occur in a structure such as 5a. In fact, when heated at 330" for 1 hr, the photoproduct gave a distillate (59%) consisting of four main components. One of these was not identified, two were found to be 1,2,3- and 1,2,4-trimethylbenzenes, and the major component (54%) was new. That this has the structure 16 is indicated by its infrared, ultraviolet, and nmr spectra.
5a
5b
16
17a
17b
Hydrogenation of this pyrolysis product gave mainly (73%) the tetrahydro derivative 17a, which gave the (31) M. Karplus, J . Chem. Phys., SO, 11 (1959); H. Conroy, Aduan. Org. Chem., 2, 311 (1960). (32) G. Ohloff. Chem. B e T . , 95, 2673 (1960); D . 5. Glass, R . S. Boikess, and S. Winstein, Tetrahedron Lett., 999 (lese),and earlier papers; W.R. Roth, Ann., 611, 10 (1964); W. R. Roth and B. Peltzer, ibid., 686, 56 (1965): R . J. Ellis and H . b l . Frey, J. Chem. Soc., 5578 (1964).
nmr33aand infrared33bspectra of authentic (j=)-l-cis2-dimethyl-trans-3-isopropylcyclopentane.Thus the stereochemistries of precursors 16 and 5a are as shown. This was confirmed by the acid-catalyzed isomerization of the photoproduct (sa) to the hydrocarbon 18. The latter was the only significant isomer isolated (4573 when 5a was left in contact with acid-washed alumina at room temperature for a short time. This cyclopentene (18) shows only end absorption in the ultraviolet spectrum, and gives infrared maxima indicative of >C=CH2 and >C=CHgroups. Its nmr spectrum is in complete accord with the assigned structure. Hydrogenation of this hydrocarbon (18)yielded two tetrahydro products (78 and 15%). The major component was identified as l-trans-2-dimethyl-cis-3isopropylcyclopentane (17b) by its infrared spectrum33b and the minor product was found to be 17a by the same method. The photoproduct 4, which is formed in 2% yield, was not obtained in quantities sufficiently large to permit detailed examination. Its nmr and ultraviolet spectra suggest a structure similar to 5a, but attempts to convert it into a degradation product of this have not yet given satisfactory results. In contrast with the findings of Ullman's group34 it seems improbable that the cyclohexadiene (2) is the principal direct precursor of 5a.35 If it were, the rate of formation of Sa should be much greater on irradiation of a-pyronene than of alloocimene, but the results given in Table I11 show that this is not the case. The triene and the cyclohexadiene, which have qualitatively similar absorption spectra, were separately irradiated in the same apparatus; virtually all the
